Circuits and techniques for voltage monitoring of a solid-state isolator

ABSTRACT

An isolation switch driver includes a control circuit to control conduction of an isolation switch and a voltage monitor circuit to monitor a control voltage coupled to the isolation switch and to generate a fault indication if the control voltage is less than a predetermined threshold. The isolator driver can drive an isolation switch coupled between a bridge network and a motor in a motor control system. The monitor circuit can monitor a voltage between a gate terminal and a source terminal of the isolation switch and include a resistive element coupled between the gate terminal and the source terminal to generate a current proportional to the VGS voltage, a current mirror to shift a level of the current to a reference potential, and a comparator to compare the level-shifted current to a threshold current.

FIELD

This invention relates generally to drivers capable of controlling andmonitoring isolation switches.

BACKGROUND

In power management applications, a bridge network is often employed tomanage and sequence power to a load, such as a motor. In safety-criticalapplications, isolation switches may be coupled between the bridgenetwork and the load to enable the load to be disconnected from powerunder certain conditions, such as dangerous or otherwise unsafeconditions. Proper operation of the isolation switches is thus necessaryto enhance safety.

SUMMARY

The present application is directed to systems and methods formonitoring an isolation switch, such as an isolation switch, thatconnects a bridge network to a motor. A motor control system can beconfigured to control operation of the motor with the bridge networkcoupled to the motor through the isolation switch. To ensure that safetyis enhanced by the introduction of the isolation switch, an isolatordriver for the isolation switch can be configured to implementdiagnostics on the operation of the isolation switch and provide a faultindication to a controller that is configured to control operation ofthe motor.

The isolator driver can include a control circuit and a voltage monitorcircuit. The control circuit can be configured to control conduction ofthe isolation switch and the voltage monitor circuit can monitor acontrol voltage coupled to the isolation switch and generate a faultindication if the control voltage is less than a predeterminedthreshold.

The isolation switch may include one or more of a solid-state relay oran electromechanical relay. The solid-state relay may be in the form ofa field effect transistor, in which case the voltage monitor circuit isconfigured to monitor a control voltage between a gate terminal and asource terminal of the field effect transistor. In some embodiments, themotor control system is configured to control operation of a three-phasemotor and the bridge network comprises a three-phase bridge network. Thecontrol voltage coupled to the isolation switch can be floating voltage.The driver may further include a charge pump to generate a supplyvoltage for the voltage monitor circuit.

According to another aspect of the present disclosure, a system formonitoring a voltage between a gate terminal and a source terminal of afield effect transistor includes a resistive element coupled between thegate terminal and the source terminal and configured to generate acurrent proportional to the voltage between the gate terminal and thesource terminal of the field effect transistor. The system furtherincludes a current mirror coupled to the resistive element andconfigured to shift a level of the current to a reference potential anda comparator configured to compare the level-shifted current to athreshold current. The current mirror may include a first leg configuredto carry the proportional current and a second leg configured to carrythe level-shifted current. The reference potential can be a groundpotential. The current mirror further includes a floating voltage sourcecoupled to the first and second legs of the current mirror.

In some embodiments, the comparator is a current comparator thatincludes a first current path configured to carry the threshold current,a second current path configured to carry the level-shifted current, anda voltage node between the first and second current paths at which avoltage is generated that is indicative of a level of the level-shiftedcurrent relative to a level of the threshold current. The field effecttransistor can be coupled between a motor winding and a motor driver andconfigured to isolate the motor winding from the motor driver.

According to another aspect of the present disclosure, a method formonitoring a voltage between a gate terminal and a source terminal of afield effect transistor includes a generating, by a resistive component,a current proportional to a voltage between the gate terminal and thesource terminal of the field effect transistor. A converter componentcan shift a level of the current to a reference potential. Thelevel-shifted current can then be compared to a threshold current. Afault indication can be generated identifying an undervoltage conditionbetween the gate terminal and the source terminal of the field effecttransistor in response to the level-shifted current being less than thethreshold current. The fault indication can be transmitted to acontroller that is configured to control conduction of the field effecttransistor.

A resistor can be provided between the gate terminal and the sourceterminal of the field effect transistor to prevent the field effecttransistor from turning on unintentionally. The resistive component mayprovide the resistor. In some embodiments, shifting the level of thecurrent includes providing a current mirror as the converter componentand providing the current mirror includes providing a first leg of thecurrent mirror to carry the proportional current and providing a secondleg of the current mirror to carry the level-shifted current, whereinthe reference potential is a ground potential.

The source terminal of the field effect transistor can be referenced toa floating voltage potential and shifting the level of the current mayinclude coupling a floating voltage source to the first and second legsof the current mirror. In some embodiments, comparing the level-shiftedcurrent to the threshold current includes providing a currentcomparator. Providing the current comparator may include providing afirst current path configured to carry the threshold current, providinga second current path configured to carry the level-shifted current, andproviding a voltage node between the first and second current paths atwhich a voltage is generated that is indicative of a level of thelevel-shifted current relative to a level of the threshold current.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features may be more fully understood from the followingdescription of the drawings. The drawings aid in explaining andunderstanding the disclosed technology. Since it is often impractical orimpossible to illustrate and describe every possible embodiment, theprovided figures depict one or more exemplary embodiments. Accordingly,the figures are not intended to limit the scope of the invention. Likenumbers in the figures denote like elements.

FIG. 1 is a circuit block diagram of a motor control system withisolation switches;

FIG. 2 is a circuit diagram block of the isolator driver of the systemof FIG. 1;

FIG. 3 is a circuit diagram of a portion of the isolator driver of FIG.2 including a gate driver and monitor circuit for VGS monitoring;

FIG. 3A is an equivalent circuit diagram of the VGS monitor circuit ofFIG. 3;

FIG. 4 is a flow diagram of a method for VGS monitoring;

FIG. 5 is a circuit diagram of a system implementing a quad-levelsignaling scheme;

FIG. 5A is a diagram of threshold levels for the quad-level signalingscheme of FIG. 5;

FIG. 6 is a circuit diagram of a system implementing a tri-levelsignaling scheme;

FIG. 6A is a diagram of threshold levels for the tri-level signalingscheme of FIG. 6;

FIG. 6B is a diagram of waveforms for a tri-level signaling scheme; and

FIG. 7 is a flow diagram of a method for the signaling schemes of FIGS.5-6B.

DETAILED DESCRIPTION

The present disclosure is directed to systems and methods forcontrolling and monitoring operation of one or more isolation switches,as may be configured for coupling between a bridge network and a load,such as a motor in a motor control system. The isolation switch(es)permit power to the motor to be disconnected in order to avoid dangerousmotor operation in the event of failure of the system or a systemcomponent. The isolation switch(es) can be controlled by an isolatordriver that includes a control circuit to control conduction of theswitch(es) and a voltage monitor circuit configured to monitor a controlvoltage coupled to the isolation switch(es). If there is insufficientcontrol voltage to accurately and reliably control conduction of theisolation switch(es), then a fault indication can be generated andtransmitted to a system controller that is configured to controloperation of the motor.

Now referring to FIG. 1, a motor control system 10 includes a bridgenetwork 12 coupled to a motor 14 through one or more isolation (orsimply isolator) switches 16, here switches 16 a-16 c. Bridge network 12can be configured to manage and sequence power to motor 14 under thecontrol of a bridge driver 17. The bridge driver 17 can, in turn, becontrolled by a controller 13 to energize and discharge the motorwindings in such a way as to start the motor, maintain the motor at aset speed, and brake the motor as commanded by the controller. Thus, thecontroller 13, as may take the form of microprocessor for example,provides control signals to the bridge driver 17 in order to ultimatelycontrol a desired operation of the motor 14. Power may be provided tothe motor control system 10 by a regulator 15 that generates a supplyvoltage VBAT.

Motor, or more generally load, 14 can have various configurations, suchas a single phase motor, a two-phase motor, or a three-phase motor as inthe illustrated system 10. Furthermore, the motor may be of varioustypes, such as a BLDC motor as in the illustrated system 10, a DCstepper motor, or even a non-motor power delivery, regeneration, andbraking load, etc. Here, bridge network 12 is a three-phase bridgeconfigured to control the illustrated 3-phase BLDC motor 14. It will beappreciated however, that other bridge network configurations can beused and can control other configurations and types of loads. As one ofmany examples, a bridge network may be provided in the form of anH-bridge to control a single phase motor.

Generally, each phase of motor 14 is associated with an independent, ordedicated isolation switch 16. Accordingly, in the illustrated 3-phaseembodiment, three isolation switches 16 a-16 c are provided, eachassociated with a respective winding of the three-phase motor 14.

Isolation switches 16 a-16 c may take various forms of switches suitableto connect and disconnect power to and from the motor 14. Examplesinclude a solid-state relay or an electromechanical relay. Theillustrated isolation switches 16 a-16 c are provided in the form ofsolid-state Field Effect Transistors (FETs) and more particularly in theform of NMOS FETs. For example, in one embodiment, isolation switches 16a-c can be coupled to motor 14 with a source terminal of each of theisolation switches 16 a-16 c coupled to the bridge network 12 and adrain terminal coupled to a respective motor winding. Alternatively,isolation switches 16 a-16 c can be coupled to motor 14 with the sourceterminal of each isolation switch 16 a-16 c coupled to the respectivemotor winding and the drain terminal coupled to the bridge network 12.

Isolation switches 16 a-16 c are controlled by an isolator driver 18.Driver 18 responds to command signal(s) from the controller 13 to openand close the isolator switches 16 a-16 c as necessary to ensure safemotor operation. Additionally, isolator driver 18 can performdiagnostics on the operation of isolation switches 16 a-16 c to ensurethey are operating correctly (i.e., in an ON state or OFF state). Incases when isolation switches 16 a-16 c are not operating properly, forexample if there is insufficient gate drive to a gate terminal of anisolation switch 16, the driver 18 can be configured to provide a faultindication to the controller 13.

Now referring also to FIG. 2, an example isolator driver 18 includes acontrol circuit 20 to control conduction of the isolation switch(es) anda voltage monitor circuit 22 to monitor a control voltage coupled to theisolation switches and to generate a fault indication if the controlvoltage is less than a predetermined threshold. In the illustratedthree-phase motor embodiment, the control circuit 20 may include threerespective control circuit portions 20 a-20 c, as shown, each configuredto couple to the gate and source terminals of a respective isolatorswitch 16 a-16 c. For example, control circuit 20 a may be coupled tothe gate terminal of isolator switch 16 a at pin GU and to the sourceterminal of switch 16 a at pin SU.

In some embodiments, voltage monitor circuit 22 is an undervoltagemonitor configured to check that the respective gate drive outputvoltage is high enough to ensure that the driven isolation switch can bemaintained in a safe conducting state. If the gate drive output voltageis less than an undervoltage threshold, a fault indication can becommunicated to the controller 13 (FIG. 1). In some embodiments, driver18 includes a plurality of voltage monitor circuits 22 a-22 c such thateach of the isolation switches 16 a-16 c is coupled to a respectivevoltage monitor circuit.

Because the isolation switches 16 a-16 c are coupled between the bridgenetwork 12 and motor 14, their source terminals are “floating” (notreferred to a fixed reference potential) and accordingly, the controlvoltage provided by the isolator driver 18 to the switches 16 a-16 c isa floating voltage. In particular, the source terminals of the switches16 a-16 c can range in voltage from below ground to above the supplyvoltage VBAT. As a result, the isolator driver 18, and in particular thecontrol circuit 20 and the voltage monitor circuit, requires a supplyvoltage that can range from below ground to greater than the supplyvoltage VBAT. To this end, the isolator driver 18 may include a chargepump 24.

In an embodiment, the system controller 13 (FIG. 1) controls theisolator driver 18 through an input 23 (e.g., ENA pin). For example,when input pin 23 is high, driver 18 drive the isolation switches to anON state. Alternatively, when input pin 23 is low, driver 18 may drivethe isolation switches to an OFF state. As will be explained, in someembodiments, the input 23 can additionally be used by the driver 18 tocommunicate with the controller 13 (e.g., can be used to communicate afault signal and/or the status of the charge pump 24).

Control circuit 20 provides a gate drive, or control signal to the gateterminal of the isolation switches. In one embodiment, each controlcircuit portion 20 a-20 c includes an N-channel power MOSFET drivercoupled to the gate terminal of the respective isolator switch andcapable of turning on the isolation switch (e.g., ON state) andmaintaining the ON state during transients on the terminals of theisolation switch. The control circuit portions 20 a-20 c can also turnoff the respective isolation switch (e.g., OFF state) and hold theisolation switch in the OFF state during transients on the terminals ofthe isolation switch.

Charge pump 24 provides a charge pump voltage VCP greater than thesupply voltage VBAT to the control circuit 20 that can be used to turnon or maintain the isolation switches 16 a-16 c in the ON statecontinuously when the phase voltage is equal to the battery voltageVBAT. With the charge pump 24, the isolator driver 18 can operate over arange of supply voltages VBAT, such as between 4.5V to about 50V,through which the driver 18 can maintain an isolation switch in an OFFstate down to approximately 4.0V. Driver 18 may operate without anyunsafe states down to a supply voltage VBAT of 0V to ensure safeoperation during power-up and power-down events. As the supply voltagerises from 0V, driver 18 can maintain the isolation switches in the OFFstate until a gate voltage to the gate terminal of the isolationswitches is sufficiently high to ensure conduction and the outputs areenabled for the isolation switch.

In one embodiment, the charge pump voltage VCP generated by the chargepump 24 can be limited to some predetermined amount greater than thesupply voltage VBAT, such as 12V greater than the VBAT voltage. Thecharge pump voltage VCP is selected to enable sufficient gate drivevoltage (e.g., gate drive voltages of at least 7.5V) to be provided tothe isolation switches 16 a-16 c for battery voltages as low as apredetermined level, such as for battery voltages down to 4.5V.

The charge pump voltage VCP may be used to power the control circuit 20and the monitor circuit 22. In one embodiment, the charge pump 24 isused to generate an auxiliary supply voltage VAUX that is provided tothe voltage monitor circuit 22, as will be explained.

The isolator driver 18 may include various additional features. Forexample, the charge pump 24 can be enabled and/or disabled through apower OK (POK) input 25 and/or an ignition (IG) input 26. The chargepump voltage VCP may be monitored to generate a charge pump undervoltageindication. The illustrative isolator driver 18 can be provided in theform of an integrated circuit or other suitable form for a particularapplication.

Now referring to FIG. 3, a portion of the isolator driver 18 (FIG. 2) isshown, including control circuit portion 20 a (or simply control circuit20 a) and voltage monitor circuit portion 22 a (or simply monitorcircuit 22 a) for controlling and monitoring a respective isolationswitch 16 a. Isolation switch 16 a is here, provided in the form of aNMOS device having a drain terminal coupled to the bridge network 12(FIG. 1), a gate terminal 31 coupled to the control circuit 20 a, and asource terminal 32 coupled to the motor 14 (FIG. 1). Monitor circuit 22a monitors the control voltage provided to the gate terminal 31 of theisolation switch 16 a by monitoring the gate to source (VGS) voltage ofthe isolation switch 16 a.

Monitor circuit 22 a can monitor the differential voltage between thegate terminal 31 and the source terminal 32 of the isolation switch 16 a(i.e., the switch control voltage) to determine if the control voltageis sufficiently high to keep the isolation switch 16 in the ON statewhen it is commanded to be in the ON state. To this end, the monitorcircuit “follows” the voltage at the source terminal 32 to determine ifthe corresponding gate terminal 31 is sufficiently higher than thesource terminal. If an under-voltage condition is detected, driver 18can generate a fault indication (e.g., signal or flag) 38 to indicatethe VGS value is under-voltage or below a predetermined voltagethreshold. In some embodiments, driver 18 can transmit or communicatethe fault indication 38 to a microcontroller (e.g., controller 13 ofFIG. 1).

As mentioned, because the isolation switch 16 a is floating, it isnecessary to drive the gate terminal 31 with a voltage higher than thesupply voltage VBAT. To this end, in some embodiments, an auxiliarysupply voltage, VAUX, 37 can supply power to monitor circuit 22. In oneembodiment, the VAUX voltage 37 is generated from either the supplyvoltage, VBAT, or the charge pump voltage, VCP, depending on certainconditions. For example, if the supply voltage VBAT is greater that thevoltage on the source terminal 31 by at least a predetermined voltage,then the supply voltage VBAT may be used to provide the VAUX voltage 37.Alternatively, if the supply voltage VBAT is not greater that thevoltage on the source terminal 31 by at least the predetermined voltage,then the charge pump voltage VCP may provide the VAUX voltage 37.

As one example, the predetermined voltage may be on the order of between6-10V. With this arrangement, the VAUX voltage 37 is established at alevel sufficient to supply only the voltage necessary to reliably drivethe isolation switch. In some embodiments, the VAUX voltage 37 is avoltage that is floating a number of volts above a voltage at sourceterminal 37, whereby the number of voltage above is selected in order toprovide sufficient voltage for the monitor circuit 22 a to operate. Inone embodiment, the VAUX voltage 37 is provided from gate terminal 31.For example, the VAUX voltage 27 may be coupled to gate terminal 31 toprovide a voltage to current mirror 29. It should be appreciated thatthe VAUX voltage 37 may be generated in a numbers of ways depending on aparticular application.

The monitor circuit 22 a includes a resistive element 28 coupled betweenthe gate terminal 31 and source terminal 32 of the isolation switch 16 ato generate a current I1 proportional to the gate to source voltage VGS.Here, the resistive element 28 is provided in the form of avoltage-controlled current source 28, such as a MOSFET device. In someembodiments, resistive element 28 is a combination of a resistor and adiode connected reference device acting as a current reference elementin current mirror 29. Resistive element 28 has a gain G0 and provides aresistance based on the gate to source voltage VGS.

A current mirror 29 is provided to shift the level of the VGSproportional current I₁ to a reference potential. Resistive element 28is coupled in series with a first leg of the current mirror 29. Thesecond leg of the current mirror 29 is coupled to a reference potential,here ground, through a further current mirror 34. Thus, thelevel-shifted current I₂ is provided in a first leg of current mirror34.

A second leg of the current mirror 34 forms part of a current comparator35 that functions to compare a mirrored version I₃ of the level-shiftedcurrent I₂ to a threshold current I₄. Threshold current I₄ can begenerated in various ways, such as with a voltage-controlled element 43coupled to a supply voltage, V_(DD), and controlled by a thresholdvoltage, Vthreshold, and having a gain G₁, as shown. Thus, the currentcomparator 35 includes a first current path configured to carry thethreshold current I₄ and a second current path configured to carry thelevel-shifted current I₃. Current comparator 35 may further include avoltage node 39 between the first and second current paths at which isprovided a voltage indicative of a level of the level-shifted current I₃relative to a level of the threshold current I₄ and thus, indicative ofthe gate to source voltage VGS being below a predetermined levelestablished by the threshold current I₄.

In some embodiments, the voltage generated at the voltage node 39 may befiltered by a filter 40 that provides a fault indication 38 indicativeof an undervoltage of the VGS voltage. In one embodiment, the filter 40may take the form of a debounce filter to ensure that a fault indicationis valid (i.e., is not a transient or false indication) beforecommunicating the fault signal 38 to the controller. Filter 40 can beimplemented using analog circuits and/or methods, digital circuitsand/or methods, or a combination thereof.

A resistance, sometimes referred to as a passive pull off resistor, iscoupled between the gate and source terminals 31, 32 of isolation switch16 a in order to prevent the isolation switch 16 a from turning onunintentionally. In the illustrative embodiment, advantageously, theresistive element 28 may perform the functionality of the passive pulloff resistor (in addition to generating the current I₁ that isproportional to the VGS voltage).

Now referring to FIG. 3A, an equivalent circuit representation of thevoltage monitor circuit 22 a of FIG. 3 is shown to illustrate amethodology for selecting the threshold current I4 (FIG. 3). A voltageVmir 36 at a first input of current comparator 35 can be consideredestablished by a resistive network 33 including a resistor divider madeup of resistors R₁ 33 a and R₂ 33 b coupled between the switch gate andsource terminals 31, 32 and a further resistor R₃ 33 c, coupled to anode between resistors 33 a and 33 b and the first leg of the currentcomparator 35, as shown. A second input of current comparator 35 is at avoltage Vmir2 45 and can be considered established by a thresholdresistor 41 coupled to the threshold voltage Vthreshold. The voltagesVmir and Vmir2 can be considered to correspond to the voltage-controlledcurrent sources 28, 43 of FIG. 3, the current I_(mir) corresponds to thecurrent I₃ of FIG. 3, and the current through threshold resistor 41corresponds to the threshold current I₄ of FIG. 3.

The Thevenin equivalent voltage across the gate and source terminals 31,32 can be expressed as:V _(thev)=((VGS)(R ₂))/(R ₁ +R ₂)  (1)And the Thevenin impedance as:R _(thev)=((R ₁)(R ₂))/(R ₁ +R ₂)  (2)By superposition, the current I_(mir) through the left side of thecurrent comparator 35 (corresponding to the current I₃ in FIG. 3) can beexpressed as:I _(mir)=(V _(thev) −Vmir)/(R ₃ +R _(thev))  (3)And the current I_(mir2) through the right side of the currentcomparator 35 (corresponding to the current I₄ in FIG. 3) can beexpressed as:I _(mir2)=(Vthreshold−Vmir2)/R _(th)  (4)

The trip point of the current comparator 35 occurs when the currentI_(mir) is equal to the current I_(mir2). The threshold resistor 41 canbe expressed as the resistance of resistor 33 c plus the Theveninresistance of equation (2) and the threshold voltage Vthreshold is equalto the Thevenin voltage of equation (1). Accordingly, the thresholdvoltage Vthreshold and the threshold resistance R_(th) can be selectedto trip the current comparator 35 at a desired undervoltage threshold ofthe VGS voltage according to the above relationships. For example, witha VGS set to 6V for example, the values of resistors 33 a, 33 b, and 33c can be chosen for a set current Imir.

In embodiments in which the voltage-controlled current source 28 acts asthe passive pull off resistor between the gate and source terminals 31,32 of the isolation switch 16 a, the value of the current Imir isselected so that the total current sourced from gate terminal 31 at fulldrive (i.e., not at a trip point) is the same as what a simple resistorpassive pull-down would normally sink. With this arrangement, thecurrent drawn by the monitor circuit 22 a is relatively small, therebyavoiding overloading the VAUX voltage and/or the charge pump voltage VCPthat powers the monitor circuit.

Now referring to FIG. 4, a flow diagram of a method for monitoring acontrol voltage provided to an isolation switch (e.g., a voltage betweena gate terminal and a source terminal of an isolation switch) isprovided. In some embodiments, the isolation switch is coupled between amotor winding and a motor driver and is configured to isolate the motorwinding from the motor driver as shown in FIG. 1.

At step 402, a resistive component is used to generate a current that isproportional to a voltage between the gate terminal and the sourceterminal of the isolation switch. In some embodiments, the resistivecomponent includes a FET, such as the element 28 of FIG. 3. In someembodiments, a resistor provided between the gate terminal and thesource terminal of the isolation switch to prevent the isolation switchfrom turning on unintentionally can additionally be used as theresistive component to generate the proportional current.

At step 404, a converter component shifts a level of the VGSproportional current to a reference potential. In some embodiments, acurrent mirror, such as current mirror 29 of FIG. 3 is used tolevel-shift the VGS proportional current.

At step 406, the level-shifted current is compared to a thresholdcurrent. In some embodiments, a comparator or current comparator isprovided to compare the level-shifted current to the threshold current,such as may take the form of current comparator 35 (FIG. 3).

At step 408, an indication of a fault is generated if the level-shiftedcurrent is less than the threshold current. A comparator can generatethe fault indication (e.g., flag) if there is insufficient controlvoltage to the gate terminal of the isolation switch. For example, theremay be insufficient voltage to keep or maintain the isolation switch inthe ON state when the isolation switch is commanded to be on. In someembodiments, the fault indication identifies an undervoltage conditionbetween the gate terminal and the source terminal of the isolationswitch in response to the level-shifted current being less than athreshold current.

The fault indication can be transmitted to various components of a motorcontrol system configured to control operation of the isolation switch.In some embodiments, the fault indication is transmitted to a controllerthat is configured to control conduction of the isolation switch, suchas controller 13 of FIG. 1. In response to receiving the faultindication, the controller can generate and transmit one or more commandsignals to components of the motor control system. For example, thecontroller can generate and transmit a command signal to cause thedriver to put the isolation switch in the OFF state. In one embodiment,a fault signal (e.g., signal 38 in FIG. 3) may be provided at a firstlogic level to indicate a VGS undervoltage condition if thelevel-shifted current I₃ is less than the threshold current I₄ and maybe provided at a second logic level to indicate that the VGS voltage issufficient to drive the isolation switch if the level-shifted current I₃is greater than the threshold current I₄

Now referring to FIGS. 5-5A, a system 50 and illustrative threshold andsignal levels for signaling between devices, such as a controller 51 anda driver 52, are shown. The driver 52 may be an isolator driver that isthe same as or similar to driver 18 of FIG. 1 and the controller 51 maybe the same as or similar to the controller 13 of FIG. 1. In theillustrated example, the isolator driver 52 includes control circuit 20a and voltage monitor circuit 22 a to control and monitor a respectiveisolation switch 16 a (FIG. 3).

Controller 51 can be configured to provide one or more command signalsto cause the driver 52 to change a state of one or more isolationswitches coupled to the driver 52. For example, the controller 51 may becoupled to a pin 53 (e.g., ENA pin) of the isolator driver 52 throughwhich the command signal is provided to control conduction of isolationswitch 16 a. In some embodiments, pin 53 is a logic level input throughwhich one or more gate drive outputs of the driver are controlled by thecommand signal from the controller.

Signaling between the controller 51 and a driver 52 can includereceiving, at the pin 53, a command signal generated by the controller.The driver, at the pin 53 coupled to the controller, can generate afault signal indicating a fault condition substantially simultaneouslywith receiving the command signal. The fault signal may indicate a faultcondition associated with the isolation switch 16 a.

In some embodiments, controller 51 can cause the driver 52 to close theswitch 16 a when the command signal is in a first state (e.g., a logichigh state) and to open the switch 16 a when the command signal is in asecond state (e.g., a logic low state). The command signal being in thefirst state to close the switch can correspond to the command signalhaving a voltage level above a first predetermined threshold (e.g.,VHI2) and the command signal being in the second state to open theswitch can correspond to the command signal having a voltage level belowa second predetermined threshold level (e.g., VLI2).

The controller 51 may include a disable switch 54 (i.e., DIS) throughwhich the command signal is generated in response to a signal from acore of the controller for example. Controller 51 can open disableswitch 54 which causes the pin 53 to be pulled up through a pull-upresistor 56 to a Vpullup voltage that is greater than the firstpredetermined threshold VHI2, thereby commanding the isolator driver 52to put the switch 16 a in the ON state. Alternatively, controller 51 canclose the disable switch 54 which causes the pin 53 to be coupled to apredetermined threshold level VL01, that is less than the secondpredetermined threshold voltage VLI2, thereby causing the isolatordriver 52 to put the switch 16 a in the OFF state.

More generally, as shown in the “Controller Out” portion of FIG. 5A, afirst range of command signal levels (here voltages from VH01 toVpullup) can cause the driver 52 to close the isolation switch 16 a anda second range of command signal levels (here, a voltage of VL01 orlower) can cause the driver 52 to open the isolation switch. Driver 52includes control circuit 20 a, as may include an inverter withhysteresis, a buffer, and a FET driver, to generate a control signal forthe isolation switch 16 a. As shown in the “Driver Input Thresholds”portion of FIG. 5A, driver control circuit 20 a can respond to commandsignal levels above the first predetermined threshold VHI2 to turn on orclose the switch 16 a and to command signal levels below the secondpredetermined threshold VLI2 to turn off or open the switch 16 a.

In some embodiments, isolator driver 52 includes an undervoltage switch55 with which the driver 52 can communicate a fault signal to thecontroller 51 through pin 53 and more specifically, can communicate morethan one fault condition. For example, in the illustrated embodiment,the driver 52 can open the undervoltage switch 55, thereby decouplingthe pin 53 from the fault switching and logic circuit 57, can couple thepin 53 to a first fault output voltage VL02 to indicate a first faultcondition based on the output of the fault switching and logic circuit57, or can couple the pin 53 to a second fault output voltage VH02 toindicate a second fault condition based on the output of the faultswitching and logic circuit 57. Thus, as shown in the “Driver Out”portion of FIG. 5A, the driver 52 can couple the pin 53 to the firstfault output voltage VL02 or to the second fault output voltage VH02. Asshown in the “Controller In” portion of FIG. 5A, the controller 51 canrespond to signal levels above a threshold VHI1 to interpret a firstfault condition and can respond to signal levels below a threshold VLI1to interpret a second fault condition. Thus, by signaling in thismanner, at least two different voltage ranges may be used to indicatetwo different fault conditions.

The voltage levels in FIG. 5A have the following relationship,VHO1>VHI1>VHO2>VHI2 and VLI2>VLO1>VLI1>VLO2. Thus, a voltage in therange from a low input level of controller 51 (i.e., VLI1) to the lowinput level of isolator driver 52 (i.e., VLI2) can result in a firststate of normal operation (e.g., a logic low state) and a voltage in therange from a high output level of isolator driver 52 (i.e., VH01) to avoltage pullup level (i.e., Vpullup) can result in a second state ofnormal operation (e.g., a logic high state). The threshold voltagelevels may be set based on a particular application and the propertiesof the components used in the respective circuits.

The first fault output voltage level VL02 to which the driver pulls thepin 53 to indicate the first fault condition is selected in order toprevent the isolation switch 16 a from being disabled. In other words,if the voltage level VL02 were too low, then the control circuit 20 awould have insufficient gate drive voltage to keep the gate terminalactive.

It will be appreciated that the above-described signaling schemeadvantageously eliminates the need for a separate dedicated connectionbetween the controller and driver, thereby reducing the number of pinsrequired for the devices 51, 52. Furthermore, the bi-directionalsignaling scheme can occur simultaneously. In other words, the driver 52is controlled by the controller at the same time that the drivercommunicates a fault condition to the controller. Additional benefit isachieved because of the “quad-level signaling” described, in which twodifferent commands can be transmitted by the controller and interpretedby the driver (i.e., to turn on or off the isolation switch 16 a) andtwo different fault signals can be transmitted by the driver andinterpreted by the controller (i.e., the first and second faultconditions). It will also be appreciated that the signaling schemesdescribed herein, while described in connection with signaling between amotor control system controller and isolation switch driver, can beapplied with similar advantages to signaling between any two devices inany type of system.

Now referring to FIGS. 6-6B, a specific example controller 61 and driver62 combination and an illustrative signaling scheme therebetween isshown. The controller 61 may be the same as or similar to controller 13of FIG. 1 and the driver 62 may be the same as or similar to isolatordriver 18 of FIG. 1. The signaling scheme illustrated in FIGS. 6-6Bdiffers from that illustrated by FIGS. 5-5A in that FIGS. 6-6Billustrate tri-level signaling whereas FIGS. 5-5A illustrate quad-levelsignaling.

Signaling components of the controller and driver are shown in FIG. 6.Controller 61 can be configured to provide one or more command signalsto cause driver 62 to change a state of one or more isolation switchescoupled to the driver 62. For example, the controller 61 may be coupledto a pin 63 (i.e., ENA pin) of the driver 62 through which the commandsignal is provided. In some embodiments, pin 63 is a logic level inputthrough which one or more gate drive outputs of the driver 62 arecontrolled by the command signal from the controller 61.

In some embodiments, controller 61 can cause driver 62 to close aswitch, such as an isolation switch coupled to a gate driver output ofdriver 62, when the command signal is in a first state (e.g., logic highstate) and to open a switch when the command signal is in a second state(e.g., logic low state). FIG. 6A shows (on the left side) example inputsignal levels to the ENA pin of the driver 62 (i.e., command signallevels generated by the controller 61). In an embodiment, the commandsignal being the first state to close the switch can correspond to thecommand signal having a voltage level above a first predeterminedthreshold (e.g., 0.7V in FIG. 6A) and the command signal being in thesecond state to open the switch can correspond to the command signalhaving a voltage below a second predetermined threshold level (e.g.,0.4V in FIG. 6A).

The controller 61 may include a disable switch 64 through which thecommand signal is generated in response to an EN signal from a core ofthe controller. For example, controller 61 can turn off or open disableswitch 64 which causes the pin 63 to be pulled up through a pull-upresistor 70 to a Vdigital voltage that is greater than the firstpredetermined threshold, thereby commanding the driver 62 to put theswitch in the ON state. Alternatively, controller 61 can turn on orclose the disable switch 64 which causes the pin 63 to be coupled to avoltage level, such as ground, that is below the second predeterminedthreshold level, thereby causing the driver 62 to put the switch in theOFF state.

Driver 62 includes control circuit 20 a, as may include a comparator 67with hysteresis to generate a control, or drive signal “ACTIVE” for anisolation switch. Driver control circuit 20 a can respond to commandsignals above the first predetermined threshold (e.g., 0.7V) to turn onor close the switch and to command signal levels below the secondpredetermined threshold (e.g., 0.4V) to turn off or open the switch.

Driver 62 can generate a fault signal and provide the fault signal tothe controller 61 through the ENA pin 63. FIG. 6A shows (on the rightside) output signal levels of the driver 62. In some embodiments, driver62 includes an undervoltage switch 65 with which the driver 62 cancommunicate a fault signal to the controller 61 through pin 63. In theillustrative embodiment, driver 62 can close the undervoltage switch 65to couple pin 63 to a fault output voltage level, V_(OLF), therebysignaling a fault condition to controller 61.

The fault output voltage level V_(OLF) may be selected to ensure thatthe driver 62 does not disable itself (i.e., does not open the isolationswitch when it is being commanded to close the switch) whencommunicating a fault condition. For example, the minimum driver outputvoltage level that indicates a fault (here 0.9V) may be selected to besome predetermined voltage above the first predetermined threshold thatcauses the driver to close the isolation switch. The difference betweenthe 0.9V minimum fault output voltage for a fault condition and the 0.7Vminimum to turn on the isolation switch may provide a guard-band toallow correct operation in the presence of noise and IR drops. It shouldbe recognized that the voltage levels provided above are for oneembodiment and that other voltage levels may be chosen based on aparticular application and properties of the components of the system.

Controller 61 can include a comparator 66 with hysteresis and a logicgate 69 (here NOR gate), to interpret the driver fault output voltageV_(OLF) as indicating a fault associated with the driven isolationswitch. More particularly, when an undervoltage condition is detected(i.e., when the fault flag UV is set) that causes the driver to closethe undervoltage switch 65, the ENA pin 63 is coupled to the driverfault output voltage V_(OLF). The threshold of comparator 66 is set totrip the comparator when the ENA pin voltage exceeds a fault threshold,here of 0.9V. The NOR gate 69 is responsive to the output of thecomparator 66 and to the inverted EN signal to ensure that thecontroller interprets a fault indication only when the disable switch 64is open (i.e., when the command signal is in a high logic state to causethe driver to close the isolation switch). With this arrangement, whenthe command signal is at a logic low level, a fault indication need notbe provided to the controller (since the command signal is calling forthe isolation switch to be off).

FIG. 6B illustrates the signaling scheme of the circuit of FIG. 6 basedon the threshold and signal levels of FIG. 6A. In a general overview,VBAT refers to a battery voltage which can be a main power supply to themotor control system in which the configuration of FIG. 6 operates, VCPrefers to a charge pump voltage of the system in which the configurationof FIG. 6 operates, UV refers to an undervoltage flag of an isolatordriver, ACTIVE refers to an output of the isolator driver, ENA refers toa pin 63 that is a logic level input to the isolator driver that can beused to control the output of the isolator driver, DIS refers to acontrol signal of a disable switch (e.g., switch 64) of a controllerconfigured to operate the ENA pin, and ENH refers to an output of acomparator 66 of the controller 61. The description below referenceselements shown in FIGS. 6 and 6A.

In the illustrative embodiment, during a power on phase indicated atpoint A, the supply voltage VBAT, is turned on, thereby energizing amotor control system such as motor control system 10 of FIG. 1.Initially, the undervoltage switch 65 may be closed to indicate that thegate to source voltage VGS voltage is too low to allow a switch coupledto driver 62 to operate properly and may stay closed until a supplyvoltage level has reached an appropriate level to operate the switch.This is further indicated by the ACTIVE signal being set low (i.e., off)in FIG. 6B. During the power on phase, the disable switch 64 is closedto pull down the ENA pin 63 in order to keep the isolation switch off.

Betweens point A and point B, the charge pump voltage Vcp steadilyincreases until it reaches an undervoltage threshold, Vcpon (e.g.,Vcp>Vcpon). When Vcp reaches Vcpon, the undervoltage switch 65 is openedand the controller 61 provides the command signal at pin 63 (e.g., ENA)at a high level to cause the driver 62 to close the isolation switch.

Between points B and C, the controller 61 may generate a command signalto turn off a switch coupled to driver 62. To this end, the controller61 may close disable switch 64 (DIS), thereby turning it on (i.e., sethigh) and in response pin 63 (ENA) is pulled down below a thresholdvoltage. Thus, the ACTIVE signal output of driver 62 is turned off(i.e., set low) to disable the gate terminal of the switch to which thedriver 62 is coupled.

Between points C and D, the controller 61 may generate a command signalto turn on a switch coupled to driver 62. For example, controller 61 mayopen disable switch 64 (DIS), thereby turning it off and in response pin63 (ENA) is pulled high above a threshold voltage. Thus, the ACTIVEsignal output of driver 62 is turned on and is fed to the gate terminalof the switch to put the switch in the ON state.

Between points D and E, if a voltage at the charge pump, Vcp, dropsbelow the undervoltage threshold, Vcpon, a timer may be started. If Vcpstays below Vcpon for the duration of an undervoltage filter time,Tcpuv, an undervoltage condition can be asserted (UV flag can be set).In response to the UV flag, controller 61 may generate command signalsto turn off a switch coupled to driver 62. In such an embodiment,controller 61 can close disable switch 64, thereby, pulling pin 63 downto ground to cause the driver 62 to turn off the isolation switch. Thus,in some embodiments, driver 62 may open undervoltage switch 65 (i.e.,set high) to indicate the undervoltage condition and pull pin 63 toV_(OLF). When the pin 63 is pulled to V_(OLF), the ACTIVE signal outputof driver 62 may be turned off

Between points E and F, when charge pump voltage Vcp rises above theundervoltage threshold, Vcpon, driver 62 can close undervoltage switch65 to indicate the undervoltage fault condition has cleared and thuspulling pin 63 above a predetermined threshold level (e.g., 1.1V).Controller 61 may respond to the fault condition clearing by turning offthe disable switch 64, thereby causing the ENA pin 63 to be pulled highand the ACTIVE signal output of driver 62 to go high and turn on theswitch.

Between points F and G, during a power off phase, the main power supplyVBAT may be turned off. As the VBAT drops, the voltage at the chargepump, Vcp, drops as well. When Vcp falls below the undervoltagethreshold Vcpon, an undervoltage condition is detected and undervoltageswitch 65 is again closed, thereby pulling pin 63 (ENA) below to faultindication threshold level V_(OLF). Controller 61 may recognize thefault indication at pin 63 and generate command signals to turn off theswitch coupled to driver 62. In response, driver 62 can turn off, thusthe ACTIVE signal output is off and the switch coupled to driver 62 isturned off.

Between points G and H, the controller 61 may generate commands toenable the switch coupled to driver 62. Controller 61 can open disableswitch 64 to pull pin 63 above a predetermined threshold level, therebycausing the ENA pin 63 to be pulled up and the ACTIVE signal to go highto turn on the isolation switch.

Now referring to FIG. 7, a flow diagram of a method for signalingbetween a controller and a driver, such as an isolator driver of FIGS. 5and 6, is shown. At step 702, a pin of the driver coupled to thecontroller receives a command signal generated by the controller. Thedriver can be configured for driving a switch (e.g., isolation switch)and the command signal can be configured to cause the driver to open theswitch when the command signal is in a first state and to close theswitch when the command signal is in a second state. In someembodiments, the first state may refer to the switch being in the ONstate and the second state may refer to the switch being in the OFFstate.

The controller may command the driver to drive the switch from the firststate to the second state or vice versa in response to a voltagedetected at a gate terminal or source terminal of the switch. Moreparticularly, in response to a fault indication (e.g., a fault signal 38as may be generated by a voltage monitor circuit 22 a of FIG. 3), thecontroller may command the driver to open the respective switch sincethe fault indication may indicate an undervoltage condition of the VGSvoltage of the switch and thus, an unsafe condition in which to operatethe motor.

At step 704, the driver, at a pin coupled to the controller, generates afault signal indicating a fault condition. In some embodiments, thefault signal is generated simultaneously with receiving the commandsignal. The fault signal may be generated in response to the voltagedetected at a gate terminal or source terminal of the switch.

All references cited herein are hereby incorporated herein by referencein their entirety. Having described preferred embodiments of theinvention, it will now become apparent to one of ordinary skill in theart that other embodiments incorporating their concepts may be used.

It is felt therefore that these embodiments should not be limited todisclosed embodiments, but rather should be limited only by the spiritand scope of the appended claims.

What is claimed is:
 1. In a motor control system including a controllerfor controlling operation of a motor with a bridge network coupled tothe motor through an isolation switch, a driver comprising: a controlcircuit to control conduction of the isolation switch; and a voltagemonitor circuit coupled between a gate terminal and a source terminal ofthe isolation switch to monitor a control voltage between the gateterminal and the source terminal of the isolation switch and to generatea fault indication if the control voltage is less than a predeterminedthreshold.
 2. The driver of claim 1, wherein the driver is configured toprovide the fault indication to the controller.
 3. The driver of claim1, wherein the isolation switch comprises one or more of a solid-staterelay or an electromechanical relay.
 4. The driver of claim 1, whereinthe control voltage coupled to the isolation switch is a floatingvoltage.
 5. The driver of claim 1, wherein the motor control system isconfigured to control operation of a three-phase motor and the bridgenetwork comprises a three-phase bridge network.
 6. The driver of claim1, wherein the driver further comprises a charge pump to generate asupply voltage for the voltage monitor circuit.
 7. The driver of claim2, wherein the controller is configured to provide a control signal tothe control circuit of the driver in response to the fault indication.8. The driver of claim 3, wherein the isolation switch comprises asolid-state relay in the form of a field effect transistor and thevoltage monitor circuit is configured to monitor the control voltagebetween the gate terminal and the source terminal of the field effecttransistor.
 9. A system for monitoring a voltage between a gate terminaland a source terminal of a field effect transistor, the systemcomprising: a voltage-controlled current source having a first terminalconnected to the gate terminal and a second terminal connected to thesource terminal and configured to generate a current proportional to thevoltage between the gate terminal and the source terminal of the fieldeffect transistor, wherein the field effect transistor is coupledbetween a motor winding and a motor driver and is configured to isolatethe motor winding from the motor driver; a current mirror coupled to thevoltage-controlled current source and configured to shift a level of thecurrent to a reference potential; and a comparator configured to comparethe level-shifted current to a threshold current.
 10. The system ofclaim 9, wherein the current mirror comprises: a first leg configured tocarry the proportional current; a second leg configured to carry thelevel-shifted current, wherein the reference potential is a groundpotential; and a floating voltage source coupled to the first and secondlegs of the current mirror.
 11. The system of claim 9, wherein thecomparator is a current comparator comprising: a first current pathconfigured to carry the threshold current; a second current pathconfigured to carry the level-shifted current; and a voltage nodebetween the first and second current paths at which a voltage isgenerated that is indicative of a level of the level-shifted currentrelative to a level of the threshold current.
 12. A method formonitoring a voltage between a gate terminal and a source terminal of afield effect transistor, the method comprising: generating, by avoltage-controlled current source having a first terminal connected tothe gate terminal and a second terminal connected to the sourceterminal, a current proportional to a voltage between the gate terminaland the source terminal of the field effect transistor, wherein thefield effect transistor is coupled between a motor winding and a motordriver and is configured to isolate the motor winding from the motordriver; shifting, by a converter component, a level of the current to areference potential; and comparing the level-shifted current to athreshold current.
 13. The method of claim 12, further comprising:generating a fault indication identifying an undervoltage conditionbetween the gate terminal and the source terminal of the field effecttransistor in response to the level-shifted current being less than thethreshold current.
 14. The method of claim 12, wherein shifting thelevel of the current comprises providing a current mirror as theconverter component and wherein providing the current mirror comprises:providing a first leg of the current mirror to carry the proportionalcurrent; and providing a second leg of the current mirror to carry thelevel-shifted current, wherein the reference potential is a groundpotential.
 15. The method of claim 12, wherein comparing thelevel-shifted current to the threshold current comprises providing acurrent comparator and wherein providing the current comparatorcomprises: providing a first current path configured to carry thethreshold current; providing a second current path configured to carrythe level-shifted current; and providing a voltage node between thefirst and second current paths at which a voltage is generated that isindicative of a level of the level-shifted current relative to a levelof the threshold current.
 16. The method of claim 13, furthercomprising: transmitting the fault indication to a controller that isconfigured to control conduction of the field effect transistor.
 17. Themethod of claim 14, wherein the source terminal of the field effecttransistor is referenced to a floating voltage potential and whereinshifting the level of the current further comprises coupling a floatingvoltage source to the first and second legs of the current mirror. 18.The method of claim 15, further comprising filtering a voltage at thevoltage node.
 19. The method of claim 16, further comprising providing aresistor between the gate terminal and the source terminal of the fieldeffect transistor to prevent the field effect transistor from turning onunintentionally.
 20. The method of claim 19, wherein thevoltage-controlled current source comprises the resistor.